Re: [PATCH 1/2] userfaultfd: Fix pmd_trans_huge() recheck race

[Qi Zheng <zhengqi.arch@bytedance.com>]

Hi Jann,

On 2024/8/13 00:42, Jann Horn wrote:
> The following race can occur:
> 
>    mfill_atomic                other thread
>    ============                ============
>                                <zap PMD>
>    pmdp_get_lockless() [reads none pmd]
>    <bail if trans_huge>
>    <if none:>
>                                <pagefault creates transhuge zeropage>
>      __pte_alloc [no-op]
>                                <zap PMD>
>    <bail if pmd_trans_huge(*dst_pmd)>
>    BUG_ON(pmd_none(*dst_pmd))
> 
> I have experimentally verified this in a kernel with extra mdelay() calls;
> the BUG_ON(pmd_none(*dst_pmd)) triggers.
> 
> On kernels newer than commit 0d940a9b270b ("mm/pgtable: allow
> pte_offset_map[_lock]() to fail"), this can't lead to anything worse than
> a BUG_ON(), since the page table access helpers are actually designed to
> deal with page tables concurrently disappearing; but on older kernels
> (<=6.4), I think we could probably theoretically race past the two BUG_ON()
> checks and end up treating a hugepage as a page table.
> 
> Cc: stable@vger.kernel.org
> Fixes: c1a4de99fada ("userfaultfd: mcopy_atomic|mfill_zeropage: UFFDIO_COPY|UFFDIO_ZEROPAGE preparation")
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>   mm/userfaultfd.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> index e54e5c8907fa..ec3750467aa5 100644
> --- a/mm/userfaultfd.c
> +++ b/mm/userfaultfd.c
> @@ -801,7 +801,8 @@ static __always_inline ssize_t mfill_atomic(struct userfaultfd_ctx *ctx,
>   			break;
>   		}
>   		/* If an huge pmd materialized from under us fail */
> -		if (unlikely(pmd_trans_huge(*dst_pmd))) {
> +		dst_pmdval = pmdp_get_lockless(dst_pmd);
> +		if (unlikely(pmd_none(dst_pmdval) || pmd_trans_huge(dst_pmdval))) {

Before commit 0d940a9b270b, should we also check for
is_pmd_migration_entry(), pmd_devmap() and pmd_bad() here?

Thanks,
Qi

>   			err = -EFAULT;
>   			break;
>   		}
>